<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 3.2 Final//EN"> <!--Converted with LaTeX2HTML 98.1p1 release (March 2nd, 1998) originally by Nikos Drakos (nikos@cbl.leeds.ac.uk), CBLU, University of Leeds * revised and updated by: Marcus Hennecke, Ross Moore, Herb Swan * with significant contributions from: Jens Lippmann, Marek Rouchal, Martin Wilck and others --> <HTML> <HEAD> <TITLE>Alternative construction</TITLE> <META NAME="description" CONTENT="Alternative construction"> <META NAME="keywords" CONTENT="vol2"> <META NAME="resource-type" CONTENT="document"> <META NAME="distribution" CONTENT="global"> <META HTTP-EQUIV="Content-Type" CONTENT="text/html; charset=iso-8859-1"> <LINK REL="STYLESHEET" HREF="vol2.css"> <LINK REL="previous" HREF="node658.html"> <LINK REL="up" HREF="node657.html"> <LINK REL="next" HREF="node660.html"> </HEAD> <BODY > <!--Navigation Panel--> <A NAME="tex2html9738" HREF="node660.html"> <IMG WIDTH="37" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="next" SRC="icons.gif/next_motif.gif"></A> <A NAME="tex2html9735" HREF="node657.html"> <IMG WIDTH="26" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="up" SRC="icons.gif/up_motif.gif"></A> <A NAME="tex2html9731" HREF="node658.html"> <IMG WIDTH="63" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="previous" SRC="icons.gif/previous_motif.gif"></A> <A NAME="tex2html9737" HREF="node1.html"> <IMG WIDTH="65" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="contents" SRC="icons.gif/contents_motif.gif"></A> <BR> <B> Next:</B> <A NAME="tex2html9739" HREF="node660.html">Sky Subtraction</A> <B> Up:</B> <A NAME="tex2html9736" HREF="node657.html">Construction of Flat Fields</A> <B> Previous:</B> <A NAME="tex2html9732" HREF="node658.html">Standard construction</A> <BR> <BR> <!--End of Navigation Panel--> <H3><A NAME="SECTION003432200000000000000"> </A> <A NAME="irac2:alternative-construction"> </A> <BR> Alternative construction </H3> For a number of projects where very deep integrations and accurate flat fields are required, and the targets are point sources or much smaller than the frame size, the sky background itself can be used to calculate the flat field. When an object is dithered on the chip, so that it exposes pixels at different positions in different integrations, the resulting 'stack' of images may be combined to produce a very accurate flat field. Since a reference image, such as the ``LAMP OFF'' frame, is not subtracted, a bias frame of the same integration time must first be subtracted from all these images. They should then all be median combined, with suitable scaling introduced to match the medians of each image to account for the variability of the sky background level. The resulting image is then normalised to obtain the flat field. <P> To subtract the bias from the science images, use <TT>COMPUTE/IMAGE</TT>. Then use <TT>COMBINE/CCD</TT> to median all the frames together with multiplicative or additive scaling to match individual frame medians (you should examine the results of both options to determine which is working best for your data). Use <TT>STATISTICS/IMAGE</TT> to calculate the image median, and <TT>COMPUTE/IMAGE</TT> to divide by this to get the normalised flat field. <P> <HR> <!--Navigation Panel--> <A NAME="tex2html9738" HREF="node660.html"> <IMG WIDTH="37" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="next" SRC="icons.gif/next_motif.gif"></A> <A NAME="tex2html9735" HREF="node657.html"> <IMG WIDTH="26" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="up" SRC="icons.gif/up_motif.gif"></A> <A NAME="tex2html9731" HREF="node658.html"> <IMG WIDTH="63" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="previous" SRC="icons.gif/previous_motif.gif"></A> <A NAME="tex2html9737" HREF="node1.html"> <IMG WIDTH="65" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="contents" SRC="icons.gif/contents_motif.gif"></A> <BR> <B> Next:</B> <A NAME="tex2html9739" HREF="node660.html">Sky Subtraction</A> <B> Up:</B> <A NAME="tex2html9736" HREF="node657.html">Construction of Flat Fields</A> <B> Previous:</B> <A NAME="tex2html9732" HREF="node658.html">Standard construction</A> <!--End of Navigation Panel--> <ADDRESS> <I>Petra Nass</I> <BR><I>1999-06-15</I> </ADDRESS> </BODY> </HTML>
